The majority of wireless communication systems require a frequency converter to translate the high frequency input signal to a lower frequency for further processing. This is accomplished by mixing the input with a locally generated frequency source. The resultant lower frequency difference term is then passed to the remainder of the receiver where it is more easily handled. Two approaches to wireless reception are available, utilizing direct or indirect conversion of the high frequency input signal to baseband. The best known example of the latter class is the superheterodyne receiver which downconverts to an intermediate frequency. In contrast, direct converters translate the RF input directly to baseband with zero intermediate frequency. Such receivers are relatively uncommon in mobile phones but common in paging systems although there is an increasing interest in applying them to mobile cellular communications.
A common problem encountered in both class of wireless receivers is effective local oscillator (LO) leakage isolation. Since frequency mixers have finite LO to RF isolation, some of the LO signal appears at the receiver input and can be observed at the antenna. If there is inadequate filtering, the magnitude of this re-radiated signal may exceed the limit set by regulatory authorities in respect of spurious emissions (FIG. 1a). Furthermore, it may have a detrimental effect on receiver performance since any LO leakage into the antenna may reflect off external objects back into the antenna and self-downconvert through the mixer to produce a spurious reception response. Such LO leakage is apparent in both indirect and direct downconverters, but is of a particular annoyance in the latter.
In direct conversion receivers the LO leakage can downconvert and contribute to a troublesome dc offset in the baseband section of the receiver following the mixer. This offset appears in the middle of the downconverted signal spectrum, and may be larger than the signal itself and much larger than thermal and flicker noise (FIG. 1b). Unless the offset is removed, the signal-noise ratio at the detector input will be low. There are a number of other sources which can contribute to an appreciable offset other than LO leakage, e.g. mismatch in the circuitry or dynamic effects such as thermal or ageing or peculiar to the wireless environment. Whatever their means of generation, these offsets are especially problematic when their magnitude changes with receiver location and orientation.
Attempts to limit the LO leakage have sought to prevent re-radiation by including additional shielding, reverse isolators and ac coupling. These physical solutions are not conducive to lightweight hand-held units used in mobile cellular communications. It has been proposed previously to cancel the dc offset after the downconversion stage using a manually controllable variable resistor or an A/D compensation technique at baseband but this is cumbersome. The first method only attempts to address the problem of static offset and neither method reduces the re-radiated local oscillator leakage.